Passive radio frequency peak power multiplier

ABSTRACT

Peak power multiplication of a radio frequency source by simultaneous charging of two high-Q resonant microwave cavities by applying the source output through a directional coupler to the cavities and then reversing the phase of the source power to the coupler, thereby permitting the power in the cavities to simultaneously discharge through the coupler to the load in combination with power from the source to apply a peak power to the load that is a multiplication of the source peak power.

BACKGROUND OF THE INVENTION

The invention disclosed herein was made under, or in the course ofContract No. AT(04-3)-515 with the U.S. Atomic Energy Commission.

The invention relates to peak power multiplication of the power from aradio frequency source, and more particularly, it relates to storingmicrowave power from the source in resonant cavities and thensimultaneously applying the power in the cavities along with the sourcepower to a load.

The usual way of increasing peak microwave power is to use additional ormore powerful microwave power sources. However, both of these approachesrequire more total power, and in either case, the sources are activedevices rather than passive and therefore relatively complex andexpensive. It is in general desirable, in either large or smallmicrowave systems, to minimize initial capital costs as well as theamount of power consumed during operation of the systems. In particular,to modify an existing very large microwave structure, such as a two-milelinear particle accelerator, to increase its peak power, it isnecessary, in order to make such a conversion practical, that thecapital cost of the modification and the ensuing operating costs beminimal and within available funding.

SUMMARY OF THE INVENTION

In brief, the present invention pertains to a radio frequency peak powermultiplier for multiplying the peak power of a radio frequency source,and includes an input waveguide for receiving radio frequency inputpower; an output waveguide for delivering radio frequency output power;and passive means coupled to the input waveguide for storing the inputpower, the passive means being operable upon cessation of the inputpower for delivering peak power to the output waveguide at a higherlevel than the input power to the input waveguide.

It is an object of the invention to multiply the peak power of a radiofrequency source by means of simple passive devices.

Another object is to minimize the capital cost of equipment used forincreasing the peak power of a radio frequency source.

Another object is to minimize the input power required for increasingthe output power of a radio frequency source.

Another object is to increase the energy of particle accelerators, radartransmitters and the like by means of simple low-cost passive devices.

Other objects and advantageous features of the invention will beapparent in a description of a specific embodiment thereof, given by wayof example only, to enable one skilled in the art to readily practicethe invention which is described hereinafter with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a prior art arrangement for supplyingradio frequency energy from a series of klystrons to successive sectionsof a linear accelerator through waveguides that connect each klystron toa corresponding accelerator section.

FIG. 2 is a cross-sectional view of an arrangement, according to theinvention, including a pair of cavities that are resonant at thefrequency of the klystrons of FIG. 1 and which may be serially connectedin each waveguide of FIG. 1 between each klystron and the accelerator bymeans of a directional coupler, for multiplying the peak power of theklystrons.

FIGS. 3A, 3B, 3C, 3D and 3E are a series of diagrams showing waveformsthat occur in the arrangement of FIG. 2.

FIG. 4 is a schematic diagram of a resonant ring that may be used inplace of the cavities of FIG. 2.

FIG. 5 is a schematic diagram of a cylindrical cavity that may be usedin place of the cavities of FIG. 2.

FIG. 6 is a schematic diagram of the arrangement of FIG. 2 adapted to bea RADAR transmitter.

DESCRIPTION OF AN EMBODIMENT

Referring to the drawing there is shown in FIG. 1 a prior art schematicdiagram of a group of klystrons 10 that are pulsed by means ofrespective klystron pulsers 11 to drive a linear particle accelerator 12through respective waveguides 14. In FIG. 2 an arrangement 16 is shownserially connected in a waveguide 14' between a klystron 10' and theaccelerator 12 for increasing the peak power from the klystron to theaccelerator, according to the invention. An arrangement similar to thearrangement 16 may be connected in each of the waveguides 14 to effect amultiplication of the peak power supplied to the accelerator 12 overeach of the waveguides 14 to thereby increase the peak energy suppliedto the accelerator. The arrangement 16 includes first and secondcavities 18 and 19 that are resonant at the frequency of the klystron10', and a directional coupler 21 having first, second, third and fourtharms 23, 24, 25 and 26, connected respectively to an input waveguide 28,the cavity 18, the cavity 19, and an output waveguide 30. A 180° phaseshifter 32 is connected to the input of the klystron 10' forperiodically reversing the polarity of power at the klystron 10' output.

It has been determined in making measurements of power radiated fromresonant cavities such as cavities 18 and 19 that the power radiatedfrom a cavity that is heavily overcoupled to a radio frequency generatorapproaches four times the power coupled to the cavities immediatelyafter the generator is switched off. Normally, this radiated powertravels as a reverse wave back toward the generator. However, byinterconnecting the generator and cavities by means of a directionalcoupler, such as the coupler 21 which may conveniently be a conventional3 db coupler, the power radiated from the cavities may be coupled to aload through the coupler, the level of power coupled to the load being amultiple of the power level from the generator, in the limit four times.In addition, by reversing the phase of the generator instead ofswitching it off, the peak power from such as the arrangement 16 andklystron 10' can be increased by up to a factor of nine, viz, n + 4n +4n = 9n, where n is the peak power level from the klystron 10' and 4n isthe peak reflected power from each cavity.

In use of the invention in conjunction with a long linear acceleratorsuch as the two-mile linear accelerator at the Stanford LinearAccelerator Center, Stanford, California, a 2.5 μsec RF pulse 33(FIG. 1) is applied from each klystron 10 through a waveguide 14 to acorresponding section of the accelerator 12. To double the energy ofsuch an accelerator, the waveguide 14 may be separated near theassociated klystron and the arrangement 16 inserted so as to be inseries with the waveguide. Power multiplication to accomplish suchenergy doubling results in a reduced pulse width since the powerradiated from each cavity decays with a time constant that ischaracteristic of the cavity filling time. However, since the energy ofthe accelerator is determined by the peak RF frequency input power, andsince many experiments are not limited by average beam power, increasingpeak RF power by reducing RF pulse width without increasing the averageinput power is a satisfactory way of increasing the energy of theaccelerator. In operation of the invention with the accelerator, thecavities 18 and 19 (FIG. 2) are made to be substantially identical andtuned to resonance at the frequency of the klystron 10'. Uponapplication of a control pulse 34 having a pulse width of 5 μsec, twicethe width of the pulse 33, to the phase shifter 32, a pulse 36 isdeveloped having a 4.2 μsec negative phase and a 0.8 μsec positivephase. A pulse similar in phase to the pulse 36 but at a higher powerlevel appears at the output of the klystron 10' and is applied throughthe input waveguide 28 to the first arm 23 of the directional coupler. Acoupling aperture 38 is centrally located within the coupler 21 and isof such dimensions as to evenly couple the power of the first phase ofthe klystron pulse through the second and third arms 24 and 25 to thecavities 18 and 19 through respective coupling apertures 39 and 40. Theresulting fields in the cavities build up during the negative phase ofthe pulse 36 until a wave of increasing amplitude is radiated from thecavities through respective coupling apertures 39 and 40. The twoemitted waves combine at the aperture 38 so as to add in the fourth arm26 to the accelerator and cancel in the first arm 23 to the klystron 10.In addition to the waves emitted from the cavities, a wave travelsdirectly from the klystron to the accelerator. This direct wave, whichis just the wave that would appear at the accelerator if both cavitieswere detuned, is opposite in phase to the combined emitted waves. If thecavities are overcoupled, the emitted waves grow in time to an amplitudewhich is larger than the direct wave. The net field at the input to theaccelerator is a wave 42 which is the sum of the direct and emittedwaves. The wave 42 goes through a phase reversal at the beginning of oneaccelerator filling time (0.8 μsec) before the end of the RF pulse fromthe klystron 10' when the phase shifter 32 reverses the phase of theoutput wave from the klystron. Immediately after this phase reversal,the emitted and direct waves add in phase in the output waveguide 30,since the emitted waves (which are proportional to the stored fields inthe cavities) cannot change instantaneously. Therefore, when theklystron phase is reversed, the field at the input to the acceleratorincreases by two units (assigning one unit to the direct wave), since atany instant the load wave is the sum of the direct and emitted waves.Following the phase reversal, the fields in the cavities (and hence theemitted waves also) decrease rapidly as the cavities try to charge up toa new field level of opposite phase. The resultant wave 42 at theaccelerator decreases also. At the end of the pulse 36, the direct wavegoes to zero and the emitted wave only is present at the accelerator. Itthen decays to zero with a time constant that is characteristic of thecavity filling time.

In order to understand the theory of an energy doubler in detail,consider the transient behavior of the reflected and stored fields for asingle resonant cavity. The field which would normally be reflected backtoward an RF generator in the case of a single resonant cavity may bedirected into a load by means of the arrangement 16. In analyzing thebehavior of a single cavity, it is convenient to consider the netreflected field as the superposition of a wave E_(e) emitted from thecoupling aperture 38 and a reverse wave E_(K) which is equal inmagnitude to an incident wave E_(i) from the generator (klystron), andwhich is reflected from the waveguide-cavity interface with a 180°-phase reversal. If at any instant the generator is turned off, thefield traveling away from the cavity is equal to E_(e), which in turn isproportional to the stored field inside the cavity at that time. If, onthe other hand, at any instant the cavity could be emptied of storedenergy (by, for example, instantaneously detuning it) then the reversewave traveling back toward the generator would be just E_(K). Byconservation of power,

    P.sub.K = P.sub.L + P.sub.c + dW.sub.c /dt,

where P_(K) is the incident power, P_(L) is the net reflected power (thepower delivered to the load in the case of the arrangement 16 shown inFIG. 2), P_(c) is the power dissipated in the cavity and W_(c) is theenergy stored in the cavity at time t. Using P_(c) = ωW_(c) /Q_(o),where Q_(o) is together with the fact that power is proportional to thesquare of the field, (P = kE²), where k in the above relation becomes

    E.sub.K.sup.2 = (E.sub.e + E.sub.K).sup.2 + E.sub.e.sup.2 /β + (2Q.sub.o /ωβ)E.sub.e dE.sub.e /dt.

A cavity coupling coefficient β has also been defined, such that kE_(e)² = βP_(c). If at any instant the generator is turned off, β is given bythe ratio of the power emitted from the coupling aperture to the powerdissipated in the cavity walls. If we now introduce the cavity fillingtime T_(c) = 2Q_(L) /ω = 2Q_(o) /[ω(1+β)], where Q_(L) is the precedingexpression can be rearranged to give

    T.sub.e dE.sub.e /dt + E.sub.e = -αE.sub.K,          (1)

where α=2β/(1 + β).

Equation (1) can now be solved for the generator waveform E_(K) shown inFIG. 3A as waveform 43. For convenience, we take E_(e) to be initiallypositive, and since initially E_(e) and E_(K) must be opposite in phase,we take E_(K) to be -1. At time t₁ the phase of the generator wave isreversed, and E_(K) = +1. At time t₂ the incident power is turned off.By solving Eq. (1), the following expressions for the emitted field inthe three time intervals A, B and C shown in FIG. 3 are obtained:##EQU1## where τ .tbd. t/T_(c), γ.tbd.α(2-e.sup.⁻.sup.τ1), e is theelectronic charge and E_(e1) and E_(e2) are the values of E_(e) at t₁and t₂. The variation in E_(e) as a function of time is shown in FIG. 3Bas waveform 44, for the case β = 5, τ₁ =2 and τ₂ = 2.4. The loadwaveform, given by E_(L) = E_(K) + E_(e), is shown in FIG. 3C aswaveform 45. The load fields are: ##EQU2## The field on a traveling-waveconstant-gradient accelerating section is given by E(z,t) =E[o,t-Δt(z)], where Δt(z) is the length of time it takes for a wave topropagate from the input of the structure to position z on thestructure. For a constant-gradient structure, in which the groupvelocity v_(g) varies linearly with z according to v_(g) (z)=v_(go)(1-gz/L), the propagation time to position z is given by ##EQU3##Defining z' = z/L, where L is the length of the accelerating structure,and integrating the above expression, we obtain

    Δt(z') = T.sub.a [ln(1-gz')/ln(1-g)]                 (4)

where T_(a) = Δt(1) = (L/gv_(go))ln[ 1/(1-g)] is the filling time forthe structure and g is a constant of the accelerator section and itdefines the variation of group velocity along the accelerator. The fieldE(z,T) along the structure is now obtained by substituting t-T_(a)[ln(1-gz')/ln(1-g)] for t in Eqs. (3). The result is: ##EQU4## wheref(z') = (1-gz')ν and ν = (T_(a) /T_(c)) [ln(1-g)].sup.⁻¹.

In using these relations, the fact must be taken into account that thereare discontinuities in the field along the accelerating structurecorresponding to discontinuities at times t_(d) =0, t₁ and t₂ in thefield as a function of time at the input to the structure. In general, afield discontinuity will occur at a position z_(d) ' along the structurefor a discontinuity at time t_(d) in the waveform at z=0, where

    z.sub.d ' = (1/g) [1 - (1-g).sup.(t.sup.'.sup.-t.sbsp.d.sup.') ]. (6)

This expression is obtained by solving Eq. (4) for z', defining also anormalized time by t'=t/T_(a), and setting Δt = t - t_(d). For example,in the time interval 0<t<t₁, the field is zero for z' > z'_(d), wherez'_(d) is obtained using t_(d) '=0 in Eq. (6). For z' < z'_(d), thefield is given by Eq. (5a).

The accelerating voltage is now obtained by integrating the field fromz'=0 to z'=1, taking into account the location of the fielddiscontinuities and using the appropriate fields given by Eqs. (5) up toand following each discontinuity. Thus the energy gain V for theinterval t₁ < t < t₂ is given by ##EQU5## where z'_(d1) is given by Eq.(6) with t_(d) '= t₁ '. The energy gain is by definition unity after onefilling time for a direct wave E_(K) = 1, which would be present withthe cavities detuned. Since f(z') is common to all of the energy gainintegrals, the calculation is simplified by defining ##EQU6## Thus,during t₁ <;0 t < t₂, ##EQU7## Similar expressions can be derived fortime intervals A and C. A plot of the normalized energy gain as afunction of time is shown in FIG. 3D as curve 46. The maximum energy isobtained after one filling time, by letting t=t₁ +T_(a) (or t'=t₁ ' + 1)and z_(d1) ' = 1. For this special case,

    V.sub.max .tbd. M = γe.sup..sup.-T.sbsp.a/T.sbsp.c [ 1 - (1-g).sup.1.sup.+.sup.ν ][ g(1+ν)].sup..sup.-1 - (α-1),

where M is the energy multiplication factor.

As a result of a number of practical and economic considerations,parameters selected that have been found to be appropriate for energydoubling of the Stanford Linear Accelerator are Q_(o) = 10⁵, t₂ 5.0 μsecand β ≈ 4.5. The energy multiplication factor for these values is 1.78as indicated in FIG. 3D during the period t₁ to t₂. The RF power outputfrom the microwave network shown in FIG. 2 is shown in FIG. 3E as acurve 47 as a function of time for the selected parameters; and, asindicated by the width of the curve 47 during the period t₁ to t₂, thereis a "pulse compression" effect due to the microwave network.

The cavities 18 and 19 may be fine tuned to resonance or completelydetuned by means of screws 49 which when tuned may be used to carry endplates 51 to points which cause the respective cavities to resonate orbe detuned. In the case where the cavities are detuned, klystron energybypasses the cavities 18 and 19 and is coupled directly from the inputwaveguide 28 to the output waveguide 30 without significant degradationof pulse shape or energy. Such detuning would be convenient where a widepulse width is sometimes required, in which cases the phase shifter 32also would be bypassed.

An alternative to the arrangement 16 (FIG. 2) is an arrangement 53 shownin FIG. 4. The arrangement 53 includes a resonant ring 55 that may becoupled to the input and output waveguides 28 and 30 by means of adirectional coupler 57. The ring 55 is made to be resonant at thefrequency of the klystron 10' for operation similar to that describedfor the arrangement 16 (FIG. 1) wherein a compressed pulse of high peakenergy is transmitted through the waveguide 30 upon reversal of phase ofthe klystron. The same effect may be obtained with still anotheralternative arrangement, arrangement 59 (FIG. 5), which includes acylindrical cavity 61 coupled to the input and output waveguides 28 and30 through apertures 63 and 64, respectively. By the use of two or moresuch coupling holes, appropriately spaced, a rotating mode is set up.For example, a TM₁₁₀ mode in a cylindrical cavity acts like a resonantring that is collapsed in radius with the center wall removed.

Another use of the instant invention is to increase the peak power of aradar transmitter. By insertion of the pulser 32, klystron 10', and thearrangement 16 including the cavities 18 and 19 and directional coupler21, between a radar pulser 66 and radar antenna 68, the peak power ofpulses from the pulser 66 to the antenna 68 over waveguide 14' may beincreased by compressing the pulses in the same manner as describedhereinbefore with respect to the accelerator 12.

While embodiments of the invention have been shown and described,further embodiments or combinations of those described herein will beapparent to those skilled in the art without departing from the spiritof the invention.

What we claim is:
 1. A radio frequency peak power multiplier formultiplying the peak power of a radio frequency source for applicationto a load, comprising:an input waveguide for receiving radio frequencyinput power; an output waveguide for delivering radio frequency outputpower; passive means coupled to said input waveguide for storing saidinput power, said means being operable upon cessation of the input powerfor delivering peak power to said output waveguide at a higher levelthan the input power to said input waveguide; a radio frequency powersource operable to emit voltage fields of first and second polarities,said power source being coupled to said input waveguide for storingpower from said power source in said passive means; and means forreversing the polarity of the voltage field from said power source forcombination with the voltage fields from said passive means to producepeak power in said output waveguide that is at a higher level than thepower from said power source.
 2. The power multiplier of claim 1,wherein said passive means includes first and second resonant cavities,a 3 db directional coupler having first, second, third and fourth arms,said first arm being coupled to said input waveguide, said second armbeing coupled to said first resonant cavity, said third arm beingcoupled to said second resonant cavity, and said fourth arm beingcoupled to said output waveguide.
 3. The power multiplier of claim 1,wherein said power source is a klystron.
 4. The power multiplier ofclaim 1, further including a load coupled to said output waveguide, saidload being a linear accelerator.
 5. The power multiplier of claim 1,wherein the peak power in said output waveguide is in the limit 9 timesthe peak power emitted from said power source.
 6. The power multiplierof claim 1, further including a load coupled to said output waveguide,said load being a radar antenna.
 7. The power multiplier of claim 2,further including means for detuning said cavities to decouple saidcavities from said input and output waveguides to enable directconduction of energy from said input waveguide to said output waveguide.